1. Field of the Invention
The present invention relates to an optical device for detecting the size and/or location of a spot on a surface.
2. Description of the Related Art
The present invention relates to an optical device, comprising an optical emitter and an optical detector, for illuminating a spot on a surface of an object, detecting the light reflected from the surface and determining the size and/or location of the spot on the surface. Although this invention has a wide variety of applications, the preferred embodiment is described in connection with an ink jet printer.
In ink jet printing, droplets of ink are selectively propelled from a plurality of drop ejectors in a print head, in accordance with digital instructions, to create a desired image on a copy surface. The print head typically includes a linear array of ejectors for conveying the ink to the copy medium, such as paper, overhead transparencies, and the like. The print head may move back and forth relative to a surface, for example, to print characters, or the linear array may extend across the entire width of a copy sheet moving relative to the print head. The ejectors typically include capillary channels or other ink passageways which are connected to one or more ink supply manifolds. Ink from the manifolds is retained within each channel until, in response to an appropriate signal, the ink in the channels is rapidly heated and vaporized by a heating element, such as a thermal resistor, disposed within the channel. This rapid vaporization of the ink creates a bubble which causes a quantity of ink to be ejected through the nozzle onto the copy sheet. When a quantity of ink in the form of a droplet is ejected from the ejector to a copy surface, the resulting spot becomes part of a desired image.
Selection of spot location for a large number of droplets is crucial to image quality in ink jet printing. If the locations of droplets ejected from the print head over the course of producing a single document vary significantly, the lack of uniformity will have noticeable effect on the quality of the image. This is particularly important in color printing, since different colored images must be overlaid on one another. A misalignment of the images can result in a defocused, staggered appearance.
In order to ensure proper alignment of the ejectors and to ensure that all the ejectors are firing properly, test patterns made up of a plurality of spots may be printed onto the copy surface in an unobtrusive location and then illuminated by an LED. A reflected signal is detected by a photodetector. If a spot is present, a different signal results from when the spot is absent. It is known to provide a mask between the copy medium and the photodetector to detect individual spots of the pattern. For example, the system disclosed in U.S. Pat. No. 4,907,013 provides a linear plurality of spots which are detected with the help of a mask having a slit approximately the same width as the spots. The spots are at an angle from the direction of motion such that detection of an X and Y location of each spot may be determined. However, the system described therein requires a plurality of spots that are visible to the naked eye and a mask slit which is approximately the same width as each spot. The longitudinal direction can only be determined by the angled disposition of the spots, and the spot size cannot be determined.
In addition, such known spot detection systems are deficient in that the effective image produced degrades over a large depth of field. This problem can be overcome by changing the operating distance between the mask and the receiver optics as the operating distance to the paper is varied. However, this solution is mechanically complex and expensive.
Accordingly, there is a need for a spot detector which is capable of detecting a single spot and which is further capable of determining both the spot size and transverse and longitudinal location. There is a further need for an improved method for a spot detector having reduced effective image degradation over a large depth of field.
These and other problems in the prior art are overcome in large part by a spot detector according to the present invention. A spot detector according to an embodiment of the present invention includes a light source with a lens arranged in the path of the light from the source and a lens disposed in the path of the light reflected from the medium surface and received by a photodetector. An opaque mask is provided and configured with at least one slit. The mask is used as a position and size detector. Depending on the size of the spot and the path of the spot relative to the mask, the photodetector is able to generate voltage pulse patterns as a function of the size and location of the spot. According to one embodiment of the present invention, a layered mask is provided wherein the optical working distance to a given mask opening is staggered by placing it on different mask substrate layers. Accordingly, different portions of the depth of field can be resolved.